1. Field of the Invention:
The present invention relates to a process for the metallization of electrically insulating, plastic shaped articles, and, more especially, to the application of strongly adherent and ductile metallic deposits onto flexible or rigid plastic substrates, employing either electrochemical and/or electrolytic means. The invention also relates to the intermediate, semi-finished and final products manufactured in accordance with such process.
2. Description of the Prior Art:
It is known to this art that the face surfaces of plastic shaped articles can be covered by or coated with any metal (nickel, copper, chromium, silver, and the like), which provides same with a glossy (most frequently), matt or satin-like metallic appearance. The following properties are a direct result thereof: electrical surface conductivity (which paves the way for major applications in the field of printed circuits); special surface appearance (which leads to applications for producing decorative elements and light reflectors, for imparting a particularly uniform surface condition or for providing good mold-release properties); improvement of surface hardness and of abrasion resistance (which leads to applications for producing plastic molds, especially for use in the foundry field for fabricating sand cores); resistance to corrosion and chemical attack of certain plastics; much reduced, or even zero, permeability to gases and vapors; thermal conductivity (which leads to applications for more easily dissipating heat from a plastic part, accompanied, as a result, by increased thermal stability and by a definitely higher heat-deformation temperature of the plastic); and the possibility of joining by welding the metallized article to one or more other objects, either made of metal or metallized plastic. The cost of production of the metallized article is typically less than that of the same object produced from whatever other metallic material. In fact, the finished plastic article is obtained upon removal thereof from the molding device (by pressing, casting, etc.) and thus is immediately ready for coating treatments.
And over the period of time of about the last two decades, considerable work has been carried out on solving these problems encountered in the coating with metal of electrically insulating substrates, such as polymeric resins, whether reinforced or not. Conventionally, metallization comprises three basic steps: (i) a surface preparation step, (ii) a sensitization and activation step and (iii) a metallization step by electrochemical means. In the surface preparation, the essential operation consists of oxidative acid pickling (or stripping), which attacks or affects the resinous surface such as to create micropores in order to promote the eventual adhesion of metallic particles thereto, which will serve as primer sites for the development of the electrochemical metallization; this operation is effected by immersion for a few minutes in a concentrated aqueous chromato sulfuric acid solution; (it should be noted that this is a rather costly process, owing to the destruction of chromic acid, which is converted by reduction to chromium (III) oxide). The subsequent sensitization and activation operations have as their object forming a discontinuous outer coating of metallic particles (or primer sites), from which, in the following step, a thin metallic coating will be produced electrochemically; sensitization can be effected by immersion in an acid solution of stannous salts; activation which follows the sensitization consists of dipping the article in an aqueous solution of a salt of a noble metal of Groups IB and VIII of the Periodic Table, (as listed in the publication Handbook of Chemistry and Physics, 45th edition, 1964-1965), usually palladium. Sensitization and activation can also be effected in a single phase by dipping the insulating substrate, for example, in an acid solution; containing both tin salts and palladium salts; this solution is the basis of an oxidation/reduction reaction between the tin and the palladium, leading to the formation of colloidal palladium, which is adsorbed on the substrate. The subsequent electrochemical metallization step (also deemed the "electroless method") consists of subjecting the thus activated substrate to the action of a metallization bath, which is an aqueous solution comprising, in particular, a water-soluble salt of the metal to be deposited and a reducing agent capable of converting the metal salt to free metal. The quality of the metallization, particularly the adhesion of the deposited metal, depends on the degree of adsorption of the metallic priming (palladium); the quality of metallization thus is closely linked to the face surface condition of the insulating substrate. Those skilled in the art that wish to employ this technique cannot, with a view towards simplification, dispense with the previous surface treatment, which is important. Moreover, they may face in this step, and also in the later sensitization and activation step, a problem of degradation of the resin in contact with the acid baths utilized. It should moreover be noted, in the sensitization and activation step, that they may also face the problem of insufficient wettability of the resin, which will hinder the contact of the bath or baths used with the substrate. Finally, the technique immediately above-described comprises numerous steps and, in addition, it is not free from shortcomings. Reference has thus far been made to three steps, but there is a fourth step, which consists of reinforcing, if required, the metal coating obtained by subsequent electrolytic metallization; this is especially the case when it is desired to provide a thick metallic coating. Electrochemical metallization is, moreover, a slow method, the rate of deposition of which is on the order of 1 .mu.m per hour; it is therefore quite useless when it is sought to provide metallic coatings, the thickness of which is to exceed approximately 20 .mu.m. In order to obtain such thick coatings, the procedure is generally to begin by effecting electrochemical deposition on the order of several .mu.m, for example, and then to carry out a second deposition electrolytically. The electrolytic method is incomparably faster, since its rate of deposition is on the order of about 50 .mu.m per hour. It too should be noted, and this circumscribes another disadvantage and drawback of the conventional technique hereinbefore outlined, that electrolytic metallization cannot properly be carried out immediately after the substrate activation step, this being due to the fact that the resulting discontinuous surface of metallic particles is not sufficiently conductive such that it is necessary, in this instance, to resort to electrochemical deposition in order to provide the required electrical conductivity.
The possibility of providing metallic primer sites on the substrate much more simply has been suggested in U.S. Pat. No. 2,690,401 which proposes the introduction into the resin, as a filler, of fine metal particles, such as, for example, aluminum particles. However, these finely divided metal particles are particularly sensitive to oxidation and same are thus rapidly covered with a non-conductive oxide layer, when exposed to air, especially during the production of the substrate; it is therefore generally appropriate to subject the face surface to be metallized, prior to any such metallization, to a gentle pickling action (generally by an acid method) such as to restore the desired values of electrical conductivity. The use of noble metals certainly eliminates the risks of oxidation, but in the majority of cases, some are too costly to be adapted for advantageous industrial application. It too has been proposed, for minimizing the risks of oxidation, to employ (non-noble) metal particles of larger dimensions. In that case, however, the same concentration by weight of metallic filler in the substrate leads to a lower surface density of primer sites and electrochemical growth will take place in modular fashion without any continuity. Finally, the distribution in the resinous substrate of metal particles in sufficient quantity for yielding a surface density of primer sites which is compatible with continuous electrochemical growth, rules out the use of this technique for the manufacture of printed circuits, since conductive bridges can result in the substrate mass thus filled.
To avoid this difficulty, it is possible to employ the technique described in U.S. Pat. No. 3,347,724 which consists of preparing a substrate filled with fine particles of non-conductive cuprous oxide and in subjecting, immediately prior to the metallization step, the surface areas of the substrate desired to be metallized, to reduction by acid treatment, such as to convert the cuprous oxide to metallic copper. It is also known, though, that reduction of cuprous oxide by acid gives rise to the formation of an unstable cuprous salt, which undergoes a change to provide, on the one hand, a cupric salt and, on the other hand, metallic copper, which is liberated according to the reaction: EQU Cu.sub.2 O+2H.sup.+ =Cu.sup.++ +Cu+H.sub.2 O
In a reduction of this type the yield of metallic copper is much lower than 50%; in fact, only half of the starting copper is capable of being converted to metallic copper and, furthermore, a portion of this reduced copper is dissolved by the acid agent employed. The result is that the density of metallic primer sites per unit surface which can be obtained is quite limited. It is certainly adequate enough to be followed by electrochemical metallization, but it is insufficient for direct metallization by the electrolytic method. In fact, this patent indicates that it is difficult or even impossible to effect metallization by electroforming. This technique thus reflects the same disadvantage as that mentioned in connection with the conventional technique outlined above which consists, for producing thick metallic deposits, of having to carry out two successive metallization steps, one by an electrochemical method and then a second one by an electrolytic method.
Cf. U.S. Pat. Nos. 3,226,256 and 3,764,280; Chemical Abstracts, 76, No. 12, 63968 m, page 317 (March 20, 1972); Chemical Abstracts, 94, No. 12, 9482 n, page 742 (Mar. 23, 1981); and German Pat. No. 2,141,759.